Mix the sourdough starter, flour and water together thoroughly, cover and leave at room temperature for 12 hours (typically overnight).
Add the cocoa powder, honey and salt and mix well. Cover and leave to rest for 10 minutes.
Knead the dough for around 10–15 seconds, cover and leave to rest for 10–15 minutes. Repeat this knead/rest process until you have a smooth, stretchy, silky dough.
Add the chocolate chips and work the dough until they are evenly distributed. Cover and leave the dough to prove for 1 hour, giving it a fold after 30 minutes.
Dust a proving basket well with flour. Shape the dough to fit, and put it in the basket, seam-side up. Cover and leave to prove at room temperature for 1–2 hours until doubled in size and the dough doesn’t spring back when pressed gently.
Heat the oven to 250°C/Fan 230°C/Gas 9+, or as high as it will go, with a baking stone or baking sheet in place. Tip the dough out onto a well-floured peel, slash the top and slide it onto the baking stone.
Turn the oven down immediately to 200°C/Fan 180°C/Gas 6 and bake for 35–45 minutes, turning the loaf round in the oven after 15 minutes to ensure even baking, and cover the top with a sheet of kitchen foil if it is browning too quickly.